Dr. Ngalula Mubenga, assistant professor of electrical engineering technology at The University of Toledo, in her lab. Credit: Dan Miller, The University of Toledo

Researchers have tapped into the benefits of both passive and active equalizers to create a cheaper and longer lasting battery pack.

A team from the University of Toledo (UT) has developed the first hybrid equalizer that combines the high performance of an active equalizer with the low cost of a passive equalizer, which could be a new energy storage solution for battery packs in electric vehicles, satellites, planes and grid stations.

”It's a game changer because we solved the weak cell issue in lithium ion battery storage for packs with hundreds of cells,” Ngalula Mubenga, PhD, an assistant professor of electrical engineering technology at UT, said in a statement. “Whenever we are talking about batteries, we are talking about cells connected in a series.

“Over time, the battery is not balanced and limited by the weakest cell in the battery,” she added.

The equalizer—dubbed the bilevel equalizer—includes cells that are grouped into sections, where each cell within a section is balanced by a passive equalizer, while the entire section is balanced by an active equalizer.

“If there are 120 cells in a battery, divide the cells into 10 groups of 12,” Mubenga said.  “Then you only need nine active equalizer units and 120 passive equalizer units using the bilevel equalizer.

“With current active equalizers, manufacturers would have to use 120 active equalizers,” she added. “For manufacturers that can't afford to use only active equalizers, the bilevel equalizer is the solution to the problem.”

After testing the new equalizer, the researchers found that the discharge capacity of lithium ion batteries increased by about 30 percent and the pack lasts longer because the cells were balanced.      

The bilevel equalizer will allow battery makers and automotive manufacturers to forego balancing the cell voltages in large battery packs using either a passive circuit that loses more energy or an active circuit, which can be 10 times more expensive.

“In spite of their significant losses, passive equalizers are used in most applications because they are relatively simple and low cost,” Mubenga said, who nearly died as a youth in Africa when her appendix burst and the hospital she was staying at in the Democratic Republic of Congo lost power for three days.

The researchers are licensing the hybrid equalizer and retrofit kit to manufacturers.

Mubenga will also present the new technology on March 7 at the Advanced Design and Manufacturing Expo at the Huntington Convention Center of Cleveland in a session entitled “Lowering the Cost of Energy Storage for E/HV and Grid Applications Using A Bilevel Equalizer for Large Li-Ion Batteries.”

The study was published in Batteries.